The present invention generally relates to total contact type image sensors, and more particularly to a total contact type image sensor which is suited for use in an image reading device which reads a large image and is provided in a facsimile machine, a computer, a word processor and the like.
An image sensor converts an image such as characters and pictures into an electrical signal, and is used in a facsimile machine and the like.
Conventionally, a charge coupled device (CCD) is generally used as the image sensor. The CCD is made of single crystal silicon and the size thereof is only in the range of 25 mm.
For this reason, when reading a document having the A4 size by use of the CCD, the overall size of the document reader becomes large because it is necessary to provide a reduction optical system and an optical path in the range of 300 mm. In addition, there is a problem in that the adjustment of the reduction optical system is complex.
FIG. 1 shows the structure of the conventional image sensor using the CCD. The image sensor shown in FIG. 1 includes a light source 2 for illuminating a document 1, a reduction lens 3 and a CCD 4.
FIG. 2 shows a contact type image sensor which was proposed to eliminate the problems of the conventional image sensor which uses the reduction optical system. The contact type image sensor shown in FIG. 2 includes an image sensor 6 which has the same size as the document 1 and is designed to read the document 1 with a 1:1 magnification. An imaging lens 5 such as a SELFOC lens and a roof mirror lens array is provided to image the characters and pictures included in the document 1 on the image sensor 6 with the 1:1 magnification.
The optical path of the imaging lens 5 is in the range of 10 to 20 mm and short. This optical path is 1/15 to 1/30 times the optical path of the image sensor which uses the CCD 4. Thus, the overall size of the document reader can be reduced by use of the contact type image sensor. In addition, there is an advantage in that it is possible to omit the expensive reduction lens 3.
However, the contact type image sensor suffers from disadvantages in that it is necessary to provide the imaging lens 5, the optical system requires adjustment, the optical path is 15 to 20 mm, and the light transmitting efficiency and the resolution of the imaging lens 5 are poor.
On the other hand, a total contact type image sensor was proposed to eliminate the disadvantages of the contact type image sensor. The total contact type image sensor includes photoelectric conversion elements which make total contact with the document via a transparent member which has a thickness in the range of 10 to 100.mu.m. No imaging lens is required in the total contact type image sensor. Hence, it is possible to reduce both the size and cost of the document reader.
In addition, because total contact type image sensor reads the document by making total contact with the document, it is possible to realize high light transmitting efficiency and high resolution.
In order to ensure a high resolution and a high signal-to-noise (S/N) ratio with the total contact type image sensor, it is necessary to illuminate the picture elements which make up the document with a high brightness and minimize the crosstalk from the adjacent photoelectric conversion elements.
Various optical systems have been proposed heretofore for the purpose of realizing the total contact type image sensor having high resolution and high S/N ratio.
FIG. 3 is a plan view showing one proposed total contact type image sensor. In FIG. 3, an array of photoelectric conversion elements 8 and an array of document illumination windows 7 are arranged parallel to each other. The document illumination windows 7 are formed in a light shielding layer 10. The photoelectric conversion elements 8 and the document illumination windows 7 are formed with a 1:1 relationship so as to improve the resolution, and the crosstalk from the adjacent photoelectric conversion elements is reduced.
FIG. 4 shows a cross sectional view of the total contact type image sensor shown in FIG. 3.
A description will now be given of the operating principle of the total contact type image sensor shown in FIGS. 3 and 4. A bundle of rays emitted from the light source 2 illuminates the document 1 via a transparent substrate 12, the document illumination windows 7 and a transparent protection layer 13. A reflected light which has an intensity dependent on the image tone of the document 1 is received by the photoelectric conversion elements 8 via the transparent protection layer 13. The photoelectric conversion elements 8 convert the received reflected light into corresponding electrical signals.
By illuminating the document 1 via the document illumination windows 7, the light from the light source 2 is blocked at portions where the document illumination windows 7 are not provided. Only the reflected light from the document 1 reaches the photoelectric conversion elements 8, and it is therefore possible to improve both the resolution and the S/N ratio.
In a case where the photoelectric conversion elements 8 are provided with a density of 8 bit/mm, for example, the photoelectric conversion elements 8 are provided with a pitch of 1/8 mm=125 .mu.m. An area c.times.d of the photoelectric conversion element 8 is approximately 100 .mu.m.times.100 .mu.m, and an area a.times.b of the document illumination window 7 is approximately 100 .mu.m.times.100 .mu.m. A thickness f of the photoelectric conversion element 8 is 1 to 2 .mu.m when an amorphous silicon thin film is used as the photosensitive material. A thickness e of the transparent protection layer 13 is 20 to 100 .mu.m.
The transparent protection layer 13 has a function of protecting the photoelectric conversion elements 8 from air and friction which is generated when the total contact type image sensor makes sliding contact with the document 1, and also a function of securing an optical path which is required to effectively receive the reflected light from the document 1. For this reason, the thickness e of the transparent protection layer 13 greatly affects the resolution and S/N ratio of the total contact type image sensor. Generally, the crosstalk from the adjacent photoelectric conversion elements decreases and the resolution is improved when the thickness e of the transparent protection layer 13 is small, but the S/N ratio deteriorates when the thickness e of the transparent protection layer 13 is too small. Hence, the thickness e of the transparent protection layer 13 is appropriately selected by balancing the resolution and the S/N ratio.
A description will be given of a case where the thickness e of the transparent protection layer 13 is selected from the point of view of improving the S/N ratio when the photoelectric conversion elements 8 are provided with the density of 8 bit/mm. When it is assumed that one side b of the document illumination window 7 and one side c of the photoelectric conversion element 8 are 100 .mu.m and the thickness f of the photoelectric conversion element 8 is 2 .mu.m, the thickness e of the transparent protection layer 13 can be calculated from the following formula (1). EQU e.apprxeq.(c/tan.theta.)+f (1)
When .theta. denotes the incident angle for a case where the picture element illuminance becomes 1/2 on the photoelectric conversion elements 8, .theta. is approximately 60.degree. from the cosine law.
Therefore, the thickness e of the transparent protection layer 13 calculated from the formula (1) becomes approximately 60 .mu.m. When the photoelectric conversion elements 8 are provided with a higher density such as 16 bit/mm, c.apprxeq.50 .mu.m and the thickness e must be set to a small value in the order of 30 .mu.m.
The reflected light from the document 1 is a diffused light which scatters in various directions. Hence, effectively receiving the reflected light becomes the condition for obtaining the high S/N ratio.
The light receiving efficiency will now be obtained for the case where the photoelectric conversion elements 8 are provided with the density of 8 bit/mm. A region where the reflected light illuminance becomes 1/2 the document surface illuminance is first obtained. When a distance between this region and the document illumination window 7 is denoted by l', l' becomes approximately 35 .mu.m from the following formula (2). EQU l'=(e-f)tan(90.degree.-.theta.) (2)
In other words, the reflected light reaches a portion 35 .mu.m on the outer side of the document illumination window 7. Accordingly, a gap g between the document illumination window 7 and the photoelectric conversion element 8 is 5 to 10 .mu.m and close. Furthermore, according to the total contact type image sensor having the structure shown in FIG. 3, the photoelectric conversion element 8 is arranged on one side of the document illumination window 7 and the photoelectric conversion element 8 only receives approximately 12% of the reflected light. For this reason, a high S/N ratio cannot be expected.
In FIG. 5, a region to where the reflected light reaches is indicated by a hatching.
When the thickness e of the transparent protection layer 13 is set to a large value, the illuminance on the photoelectric conversion element 8 becomes high but the crosstalk to the adjacent photoelectric conversion elements 8 increases as shown in FIG. 5. As a result, it is impossible to obtain a high resolution.
For the above described reasons, it is difficult to obtain high resolution and high Ser. No. ratio with the total contact type image sensor shown in FIG. 3.
In addition, the total contact type image sensor shown in FIG. 3 requires the transparent protection layer 13 which has the thickness of 50 to 100 .mu.m in order to direct the reflected light from the document 1 on all of the photoelectric conversion elements 8. Consequently, the resolution deteriorates when an attempt is made to obtain a high S/N ratio. However, if the photoelectric conversion elements 8 were arranged to surround the document illumination windows 7, the transparent protection layer 13 will become thin, the crosstalk to the adjacent photoelectric conversion elements 8 will decrease, and it will be possible to improve both the resolution and the S/N ratio. A Japanese Laid-Open Patent Application No.59-48954 proposes such a total contact type image sensor.
FIGS. 6 and 7 are a cross sectional view and a plan view respectively showing the total contact type image sensor proposed in the Japanese Laid-Open Patent Application No.59-48954. As shown in FIGS. 6 and 7, this proposed total contact type image sensor has the document illumination window 7 provided within the photoelectric conversion element 8. According to this structure of the total contact type image sensor, it is possible to suppress the crosstalk to the adjacent photoelectric conversion elements 8 and improve the light receiving efficiency without deteriorating the resolution. When the photoelectric conversion elements 8 are provided with the density of 8 bit/mm, one side c (or d) of the photoelectric conversion element 8 is approximately 100 .mu.m, one side a (or b) of the document illumination window 7 is approximately 10 to 60 .mu.m, and the thickness f of the photoelectric conversion element 8 is approximately 1 to 2 .mu.m when the amorphous silicon thin film is used as the photosensitive material.
Based on the formula (1), the thickness e of the transparent protection layer 13 can be calculated as follows. EQU e.apprxeq.(c/2).times.(1/tan.theta.)+f.apprxeq.30 .mu.m
Therefore, the thickness e can be made thin. For this reason, there is an advantage in that the transparent protection layer 13 can be formed uniformly from SiO.sub.2, Si.sub.3 N.sub.4 and the like using thin film techniques such as sputtering, plasma chemical vapor deposition (CVD) and evaporation.
Next, a region where the reflected light illuminance becomes 1/2 the document surface illuminance will be obtained similarly as in the case of FIG. 3. From the formula (2), the distance l' between this region and the document illumination window 7 is approximately 20 .mu.m in this case. It is possible to illuminate the photoelectric conversion elements 8 in their entirety when one side a (or b) of the document illumination window 7 is 60 .mu.m, as may be seen from FIG. 8. As shown in FIG. 8, the light receiving region is large compared to that of the total contact type image sensor shown in FIG. 3, and the crosstalk to the adjacent photoelectric conversion elements 8 is small.
However, according to the total contact type image sensor shown in FIG. 6, there is a limitation in that the document illumination window 7 must be provided within the photoelectric conversion element 8. For this reason, when the density of the photoelectric conversion elements 8 is increase to 16 bit/mm, for example, one side c (or d) of the photoelectric conversion element 8 becomes approximately 50 .mu.m and one side a (or b) of the document illumination window 7 must be approximately 20 to 30 .mu.m. Consequently, the area of the document illumination window 7 becomes approximately 36% of the area of the document illumination window 7 shown in FIG. 3, and the S/N ratio becomes poor due to the decrease in the document surface illuminance.
Furthermore, as may be seen from FIG. 6, a stepped portions are formed on the surface of the transparent protection layer 13 depending on the structure of the photoelectric conversion elements 8. In other words, the surface of the transparent protection layer 13 rises by 1 to 2 .mu.m at the portion corresponding to the photoelectric conversion element 8 and falls at the portion corresponding to the document illumination window 7.
The surface portion of the transparent protection layer 13 is shown in FIG. 9. In FIG. 9, the document transport direction is indicated by an arrow DT. When the document 1 makes sliding contact with the concavoconvex surface of the transparent protection layer 13, foreign particles 18 including dust particles, fine fragments of paper and ink residue from ball-point pens easily adhere on stepped portions 13a and 13b of the transparent protection layer 13. As a result, the optical output and the resolution of the total contact type image sensor deteriorate when such foreign particles 18 adhere on the surface of the transparent protection layer 13.
On the other hand, a total contact type image sensor shown in FIG. 10 is proposed in a Japanese Laid-Open Patent Application No. 58-38061. According to this proposed total contact type image sensor, the document illumination windows 7 have the shape of slits extending parallel to the document transport direction. The foreign particles are less likely to adhere on the surface of the transparent protection layer 13 when compared to the total contact type image sensor shown in FIG. 6, but there is still the limitation to provide the document illumination window 7 within the photoelectric conversion element 8. For this reason, the document surface illuminance is poor due to the small document illumination window 7 and a high S/N ratio cannot be obtained, similarly as in the case of the total contact type image sensor shown in FIG. 6.
Therefore, according to the prior art, it is difficult to realize a total contact type image sensor having high resolution and high S/N ratio. In addition, stepped portions are formed on the surface of the transparent protection layer and foreign particles easily adhere on the stepped portions thereby deteriorating the resolution and S/N ratio of the total contact type image sensor.